So you have two boys and are looking for a girl to cuddle. Maybe you have three girls and are looking for a bit of testosterone in the house to even things out. Perhaps the idea of the “balanced” family appeals to you: one girl, one boy. Whatever the reason, the idea of choosing the gender of your next child can be appealing.

Some couples choose the scientific route: gender selection done in a lab by separating the male and female sperm, sorting them by the weight of the DNA. This new technique offers an 85 percent chance of conceiving a girl and a 65 percent chance of conceiving a boy. Ethical and cost considerations generally lead couples who want to have some influence over the gender of their future child to more natural, less high-tech solutions.

Natural gender selection requires an understanding of a woman’s fertility cycles. Unlike a man, who is fertile every day of his life once he reaches puberty, a woman is fertile for only a few days of her cycle, or roughly 20 percent of the time. Because a woman can only get pregnant when she is fertile, it takes a bit of knowledge about the woman’s cycle to help choose the gender of the child.

To determine when she ovulates, a woman can chart her basal body temperature, chart her cervical fluid, use ovulation predictor kits, or any combination of the three. Statistically, most women ovulate mid-cycle, about 12 to 16 days before menstruating.

The trick to gender selection is learning about sperm. Male sperm (or Y chromosome-carrying sperm cells) are generally faster, but the downside is that they tend to die more quickly than female sperm (X chromosome-carrying sperm cells). Some researchers theorize this is because the male sperm burns out before the female sperm; others believe that the vaginal/cervical environment is hostile to male sperm.

If you want to try for a girl, have relations as soon as a woman’s body signals that she is fertile. Have intercourse every night until two days prior to ovulation. This gives time for the male sperm to die off, but still leaves plenty of the female sperm behind when the egg arrives.

To try for a boy, have intercourse the day before and the day that you ovulate. This ensures that the faster male sperm are still around in greater quantities when the egg is released, giving them better odds of reaching the egg first.

Of course, no method is foolproof (even in the lab, they miss by 15 to 35 percent), so try to go into your experiment with the idea that no matter what your child’s gender, you’ll love it. When all is said and done, all that matters is a healthy baby.